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Dissociative ionization of O2 and N2 by electron impact — N+ and O+ kinetic energies and angular distributions
Volume
22, number
I
CHEhlICAL
PIIYSICS
LETTERS
15 September
1973
DISSOCIATIVE IONIZATION OF 0 AND N BY ELECTRON IMPACT N+ AND 0’ KINETIC ENERGIES iND ANhJLAR DISTRIBUTIONS* J.A.D. STOCKDALE and Liliana DELEANU** lleallir fh!‘sics DilSisiorl, Oak Ridge h’arioual Laboralorj,, Oak Ridge, Tctrr~cssee 37830, USA Received
25 June
1973
An apparatus containing cross nlolecular and pulsed electron beams has been used to obtain distributions in kinetic through dissociative ionization of N2 and 0, by impact of cncryy 2nd angle of fast ( ,> 0.5 cV) positive ions producrd 50 to 200 cV electrolls. Four m;lin 0’ ion groups arc observed with peak energies of 0.8. 2.0, 3.0, and 5.0 eV. T\vo main Nf groups peaking at 2.0 and 3.0 cV are seen. Angular distribu Lions of both N+ and O+ ions arc essentially isotropic for electron-bcnm-ion detcution angles from 30’ to 1 IO’.
I. llltroduction
In 1935 and 1941 Sasaki and Nakao [l] reported measurements of electron-impact-irldtlccd dissociative ionization of H,. They found that the angular distribution of prorons with energies between 5 and 9 eV from impact of approximateiy 100 cV electrons on f12 was not
isotropic.
only
Aparr
measurements
from which
this have
pioneering given
work, nearly
the
comple
tc
kinematic information - both energy and angular distributions - on the ions produced in this process have been those of Dunn. Kicffer, van Brunt (DKB), and their colleagues. They have reported measurements on H-, [‘_I and N, [3] and an O+ kinetic energy distribution from 0, [S] Since it has been shown experimentally that dissociative ionization may contribute :! large fracticn of the total ionization cross section [5], it is important that further studies be made, preferably by an independent method. Ln this letter we report preliminary results of a study of dissociative ionization of N, and 01 by electron impact. 2. Experimental The apparatus
is shown schematically
in fig. 1. It
*
Research sponsored by the U.S. Atomic Energy Commission ** under contract with the Union Carbide Corporation. Graduate student from the University of Tennrssce, Knox. ville. Tennessee, USA.
.:
‘.
employs crossed molecular and pulsed electron beams, and product ion energies are measured by a time-offlight method*. Both versions of the DKB apparatus [7] employed a differentially pumped extended gas target and magnetic momentum analysis of the products In the present work the electron beam could be rotated in the horizontal plane about the vertical gas beam through an anplar range 01-30” 10 110” with respect to the fixed channeltron ion detector. Two features helped to maintain a constant beam-beam intersection geometry. First, the centrally located collimated hole array rotated with the electron gun in the horizontal plane and, second, the focussing of the steerable electron beam (quadrupole elements in tile anode structure of the gun) was checked by concentric collectors located in -he Faraday cup beam trap. The electric field free ion drifr region was defined by a shielded grid system located in front of the channeltron. This contained a variable potential grid sandwiched between two grounde grids and served two other functions. It provided a of shecking the neutral metastable contribution since voltages applied to the central grid which are adequate to repel the ions are, in general, not sufficient to quench metastables, and it also provided a crude direct means of obtaining ion energy distributions by a retarding analysis. The metastable atom contributions to the present results appeared to be negligible. The apparatus * Freund rrediods
[6] has summarized applied to neutral
a number of similar time-of-night particle velocity measurement.
Volume
22, number
I
CHI-MICAL
PIIYSICS
was operated in the pulse counting mode. A rime-to. pulse-height converter (TTPHC) WZIStriggered by the electron beam pulse and was stopped by the amplified channeltron ion detection pulse. The output of the TTPHC was fed into one half of the memory of a 5 12 chan;?el pulse height analyzer, the other half being reserved for storage of background spectra obtained wil_h the gas beam turned off. Difference time-of-flight spcctra corresponding to electron-beam-gas-beam interactions were obtained by subtraction and printed out automatically. A full account of the apparatus and an extended account of measurements on 0, and N2 incorporating mass analysis will be published elsewhere.
3. Results The results to follow are all confined to the electron energy (I?‘,) range between 50 and 300 eV.
LETTI’KS
15 Scprrmbcr
1973
Fig. 2 shows kinetic energy spectra of it’+ ions from impact of 80 and 150 eV electrons on N,. It was obtained by a computer point-by-point conversion OC the original time-of-tlight spectra. The results are compared with the measurements of Tatc and Lazier [i] which were calculated by differentiation of retzcding measurements from a Lozier tube, and with the measurements of Kieffer and van Bnint [3] The chief features of the present results arc h‘+ ion peaks with kinetic en. ergies near 2.0 and 3.0 eV, typified by the two parts of fig. 2. These peaks are somewhat mare narroiv than the other measurements, and there appears to be seine disagreement on the peak posltlons. An addition of 0.6 to 0.8 eV to our ion kinetic enera scale would bling the peak positions into reasonable agreement \vith the other spectra, but there are a number of factors mditatiiig against this. Of/O2 energy distributions (fig. 4) ob 205
Volume 22,
CIiEMICAL PtlYSlCS LETTERS
number 1
-.
TATE AND LOZIER
((9323
KIEFFER
BRUNT
AND
MN
1973
l1967)
THIS WORK
goi
32
15 September
I
I
I
I
I
I
I
10
12
I4
16
0 0
2
4
6
8
10
0 ION
Fig. 2. N+iNz cner_ey spuctra at 80 and
150
2
6
4
8
ENERGY (&I
eV clectrun energ!‘. Electron-beam-ion
tamed under the same conditions show good agreement in O+ peak kinetic energies with other workers, and the position of the dip at 0.9 eV apparent in our 150 eV N+/N, spectrum agrees with observations reported by Kieffer and van Brunt [3]. They were compelled to use a drawout field to examine ions of less than 1 eV kinetic energy and so did not obtain an accurare shape for the ion enerey distribution in this region. However, they did find evidence for a pronounced dip in the Nf ion current near 1 eV. This is not consistent with Tate and Lazier’s result but agrees with the present work. Possible further support for our kinetic energy scale lies in the fact that there is some evidence from Tate and Lazier [7] for an N+ peak near 3 eV for electron energies from 60 to 85 eV. This is the region in which we observe this peak. Lastly,
detection angle. 90’.
tially isotropic, though Kieffer and van Brunt observed a small increase in the ion intensity in the forward and backward directions with respect to the electron beam. Anguiar distributions obtained for ions of 1, 2: and 4 cV in the present work were also isotropic within the accuritcy
of the data.
1.6
,
KIEFFER VAN
the time-of-flight method should be less subject to errors from contact potentials than either of the other methods. Fig. 3 shows an angular distribution of 2.9 eV Nf ions, obtained at an electron energy of 60 eV, together with the data of Kieffer and van Brunt [3] _ Both measurements show the angular distribution to be substan206
THIS L
011
1
AND
BRUNT
(19671
WORK
’ 20
/ 60
40 ELECTRON
Fig. 3. Angular
00
BEAM-ION
distribution
DETECTOR
100
I
120 ANGLE
(degl
of 2.9 eV N+ ions from
60 eV electrons on Na (0, electron gun on north +, electron gun OP south side of detector).
140
impact
of
side of detector;
Volume
22. number
I
I
1
I
CHEXIICAL
I
I
I
I
I
PHYSICS
LETTERS
IS Srptentbzr
1973
I
7
J
20
40 ELECTKON
60. BEAM-ION
EC 100 120 :I;0 CETECTOR PNCLE (c?ql
(60
Fig. 5. Angular distribution of 0.8, 2.0, 3.0, and 5.0 cV O’ions from impact of 50 cV electrons on 02. Points sreaverugcs of data obtained \vith the cleFtron gun located on :hc t\vo symmetric opposite sides of the apparnruq with respect to the ion dctec:or.
of the B ‘2; state of 0:. Dunn’s selection rules for dipolar transitions between states of diatonic molecules [9] imply that near threshold * transitions from the ground 3Ti state of 0, to a ‘Xi state are sllowed for orientations of the internuclea;axis of the oxygen molecules both parallel to and perpendicular to the momentum of the input electrons. This would produce an isotropic angular distribution. Danby and Eland [lo] have recently made photoelectron-photoion coincidence measurements on 0,. They observed vibrational structure in the photoelectron spectrum corresponding to predissociation of the 3XS state and
socintion
0
,
2
3
4 FRAGMENT
5
6
ENERGY
7
e
9
10
(eV)
Fig. 4. Energy spcctr~~of osygpn fragments from dissociation of 02. A. This work. 50 cV electron impact. O+dctcctcd; B, Kicffcr ct al. [4]. 35 eV electron impact; C, Prcund [ 61, 39 cV clectrcn impact, high Rydbergs detected; D, Doolittlc ct al. [ 81, 34.6 CV photons, 0’ detected.
3.2. 0+/o,
were
Fig. 4 sl~ows an energy
spectrum
o f0 ’ ions obtained
at an electron ener_p of 50 eV. It is compared with an energy spectrum of Of ions obtained by Kieffer et al. [4] using the DKB apparatus, energy spectra of Of ions produced by photoionization [8] , and 0 atoms in high Rydberg states produced by electron impact on O2 [6]. All four sets of experiments show qualitatively similar general features and good agreement in the kinetic energies corresponding to the peak positions. Fig. 5 shows angular distributions of the four O+ peaks within the 30” to 115” region for E, = 50 eV. All four peaks show substantially isotropic distributions. Both Doolittle et al. [8] and Freund [6] have considered possible states OF the 0: core leading to the observed dissociation fragments-All the measurements, including the isotropic angular distribution, and our own appearance potential measurements appear consistent with the suggestion [8] that the 0.8 eV peak arises from predis-
able
to show
rhnt
the
photoions
in cokcidencc
possessed kinetic energies consistent with this prcdissociation. Assignments of 0; levels corresponding to tiic 7 and 3 eV peaks are more speculative. The states c4C; and 211,111 are the only likely candidates [5. 81. Transitions to the c4C; state would, near threshold, tend to produce an Of angular distritution peaked in the forward and backward directions with respect to the electron beam [9] if dissociation were immediate. Iiowcver. there is evidence from a number of sources (see ref. [6] ) for predissociation from this state above its first vibrational level. If the times involved in this were long enough (of the order of a rotation time), a more isotropic distribution would result. Excitation of the III,, state on with
these
electrons
* That is, for inpul electron energies such that the wavrfunctions of the two outgoing eleclrons from the process e + 0: O+ + 0 + c + e are nearly spherically symmetric
207
-
‘Volume 22. number
1
CHEhlICAL PHYSICS LETTERS
theother hand should lead to an 0’ angular distribution peaking near 90”, provided predissociation does not occur. If the isotropic distribution observed here persists to near threshold for excitation to these two states (approximately 25 eV). wc must invoke one of three possibilities: (i) involvement of both states in such a way that an isotropic O+ djstribution results *; (ii) predissociation involving iong cross-over times to repulsive states; or (iii) inclusion of non-dipolar terms in calculating the dependence of transition probability on tile orientation of the molecule with respect to the electron beam. Angular distribution measurements closer to threshold are being pursued. Freund [G] has suggested that the approximately 5 eV kinetic energy neutral oxygen peaks observed by him in time-of-flight spectra may arise from initial ization of 0, to the *Xi and/or 4Zi states formed
ionby
removal of the 0~23 electron. From Dunn’s arguments [9], we might expect an isotropic O+ angular distribution, such as that observed! From excitation to either of these states.
* Accurate measurements ~C;LTthe thresholds may permit a test of rhis possibility.
for these states
15 September
1973
Acknowledgement We are grateful to Dr. R.J. van Brunt for several very helpful discussions and to Drs. R.N. Compton and C.E. Klots of this laboratory for their constant support and encouragement.
References [l] N. Snsaki and T. Nakao, Proc. Imp. head. (Tokyo) 11 (1935) 138; 17 (1941) 75. [2] G.H. Dunn and L.J. Kicffer, Phys. Rev. 132 (1963) 2109; R.J. van Brunt and L.J. Kicffer, Phys. Rev. A 2 (1970) 1293. [3] L.J. Kicffcr nnd R.J. van Brunt. J. Chcm. Phyr. 4b (1967) 272B. r41 L..y. Kieffer, G.Af. La\~rcncc ancl J.hI. Slnrer, Proceedings VII ICPEAC. Amsterdam (1971) p. 574. (51 D. Rapp. P. Erqlandcr-Golden and D.D. Brigha, J. Chcm. Phys. 42 (1965)4081; R.J. van Brunt, R.C. Hirsch and W.D. Whitehead, RuII. _i,m. Phys. sot. 17 (1972) 1145. 161 R.S. Freund, J. Chem. Phys. 54 (1971) 3125. [71 J.T. Tate and W.15’.Lazier. Phys. Rev. 39 (1932) 254. 181 P.II. Doolittle, R.I. Schoen and K.E. Schubert. J. Chern. Phys. 19 (1968) 5108. G.11. Dunn. Phys. Rev. Lcttcrs 8 (1962) 62. Cl. Danby and J.H.D. Eland. Intern. J. Mass Spectrom. Ion Phys. 8 (1972) 153.
22, number
I
CHEhlICAL
PIIYSICS
LETTERS
15 September
1973
DISSOCIATIVE IONIZATION OF 0 AND N BY ELECTRON IMPACT N+ AND 0’ KINETIC ENERGIES iND ANhJLAR DISTRIBUTIONS* J.A.D. STOCKDALE and Liliana DELEANU** lleallir fh!‘sics DilSisiorl, Oak Ridge h’arioual Laboralorj,, Oak Ridge, Tctrr~cssee 37830, USA Received
25 June
1973
An apparatus containing cross nlolecular and pulsed electron beams has been used to obtain distributions in kinetic through dissociative ionization of N2 and 0, by impact of cncryy 2nd angle of fast ( ,> 0.5 cV) positive ions producrd 50 to 200 cV electrolls. Four m;lin 0’ ion groups arc observed with peak energies of 0.8. 2.0, 3.0, and 5.0 eV. T\vo main Nf groups peaking at 2.0 and 3.0 cV are seen. Angular distribu Lions of both N+ and O+ ions arc essentially isotropic for electron-bcnm-ion detcution angles from 30’ to 1 IO’.
I. llltroduction
In 1935 and 1941 Sasaki and Nakao [l] reported measurements of electron-impact-irldtlccd dissociative ionization of H,. They found that the angular distribution of prorons with energies between 5 and 9 eV from impact of approximateiy 100 cV electrons on f12 was not
isotropic.
only
Aparr
measurements
from which
this have
pioneering given
work, nearly
the
comple
tc
kinematic information - both energy and angular distributions - on the ions produced in this process have been those of Dunn. Kicffer, van Brunt (DKB), and their colleagues. They have reported measurements on H-, [‘_I and N, [3] and an O+ kinetic energy distribution from 0, [S] Since it has been shown experimentally that dissociative ionization may contribute :! large fracticn of the total ionization cross section [5], it is important that further studies be made, preferably by an independent method. Ln this letter we report preliminary results of a study of dissociative ionization of N, and 01 by electron impact. 2. Experimental The apparatus
is shown schematically
in fig. 1. It
*
Research sponsored by the U.S. Atomic Energy Commission ** under contract with the Union Carbide Corporation. Graduate student from the University of Tennrssce, Knox. ville. Tennessee, USA.
.:
‘.
employs crossed molecular and pulsed electron beams, and product ion energies are measured by a time-offlight method*. Both versions of the DKB apparatus [7] employed a differentially pumped extended gas target and magnetic momentum analysis of the products In the present work the electron beam could be rotated in the horizontal plane about the vertical gas beam through an anplar range 01-30” 10 110” with respect to the fixed channeltron ion detector. Two features helped to maintain a constant beam-beam intersection geometry. First, the centrally located collimated hole array rotated with the electron gun in the horizontal plane and, second, the focussing of the steerable electron beam (quadrupole elements in tile anode structure of the gun) was checked by concentric collectors located in -he Faraday cup beam trap. The electric field free ion drifr region was defined by a shielded grid system located in front of the channeltron. This contained a variable potential grid sandwiched between two grounde grids and served two other functions. It provided a of shecking the neutral metastable contribution since voltages applied to the central grid which are adequate to repel the ions are, in general, not sufficient to quench metastables, and it also provided a crude direct means of obtaining ion energy distributions by a retarding analysis. The metastable atom contributions to the present results appeared to be negligible. The apparatus * Freund rrediods
[6] has summarized applied to neutral
a number of similar time-of-night particle velocity measurement.
Volume
22, number
I
CHI-MICAL
PIIYSICS
was operated in the pulse counting mode. A rime-to. pulse-height converter (TTPHC) WZIStriggered by the electron beam pulse and was stopped by the amplified channeltron ion detection pulse. The output of the TTPHC was fed into one half of the memory of a 5 12 chan;?el pulse height analyzer, the other half being reserved for storage of background spectra obtained wil_h the gas beam turned off. Difference time-of-flight spcctra corresponding to electron-beam-gas-beam interactions were obtained by subtraction and printed out automatically. A full account of the apparatus and an extended account of measurements on 0, and N2 incorporating mass analysis will be published elsewhere.
3. Results The results to follow are all confined to the electron energy (I?‘,) range between 50 and 300 eV.
LETTI’KS
15 Scprrmbcr
1973
Fig. 2 shows kinetic energy spectra of it’+ ions from impact of 80 and 150 eV electrons on N,. It was obtained by a computer point-by-point conversion OC the original time-of-tlight spectra. The results are compared with the measurements of Tatc and Lazier [i] which were calculated by differentiation of retzcding measurements from a Lozier tube, and with the measurements of Kieffer and van Bnint [3] The chief features of the present results arc h‘+ ion peaks with kinetic en. ergies near 2.0 and 3.0 eV, typified by the two parts of fig. 2. These peaks are somewhat mare narroiv than the other measurements, and there appears to be seine disagreement on the peak posltlons. An addition of 0.6 to 0.8 eV to our ion kinetic enera scale would bling the peak positions into reasonable agreement \vith the other spectra, but there are a number of factors mditatiiig against this. Of/O2 energy distributions (fig. 4) ob 205
Volume 22,
CIiEMICAL PtlYSlCS LETTERS
number 1
-.
TATE AND LOZIER
((9323
KIEFFER
BRUNT
AND
MN
1973
l1967)
THIS WORK
goi
32
15 September
I
I
I
I
I
I
I
10
12
I4
16
0 0
2
4
6
8
10
0 ION
Fig. 2. N+iNz cner_ey spuctra at 80 and
150
2
6
4
8
ENERGY (&I
eV clectrun energ!‘. Electron-beam-ion
tamed under the same conditions show good agreement in O+ peak kinetic energies with other workers, and the position of the dip at 0.9 eV apparent in our 150 eV N+/N, spectrum agrees with observations reported by Kieffer and van Brunt [3]. They were compelled to use a drawout field to examine ions of less than 1 eV kinetic energy and so did not obtain an accurare shape for the ion enerey distribution in this region. However, they did find evidence for a pronounced dip in the Nf ion current near 1 eV. This is not consistent with Tate and Lazier’s result but agrees with the present work. Possible further support for our kinetic energy scale lies in the fact that there is some evidence from Tate and Lazier [7] for an N+ peak near 3 eV for electron energies from 60 to 85 eV. This is the region in which we observe this peak. Lastly,
detection angle. 90’.
tially isotropic, though Kieffer and van Brunt observed a small increase in the ion intensity in the forward and backward directions with respect to the electron beam. Anguiar distributions obtained for ions of 1, 2: and 4 cV in the present work were also isotropic within the accuritcy
of the data.
1.6
,
KIEFFER VAN
the time-of-flight method should be less subject to errors from contact potentials than either of the other methods. Fig. 3 shows an angular distribution of 2.9 eV Nf ions, obtained at an electron energy of 60 eV, together with the data of Kieffer and van Brunt [3] _ Both measurements show the angular distribution to be substan206
THIS L
011
1
AND
BRUNT
(19671
WORK
’ 20
/ 60
40 ELECTRON
Fig. 3. Angular
00
BEAM-ION
distribution
DETECTOR
100
I
120 ANGLE
(degl
of 2.9 eV N+ ions from
60 eV electrons on Na (0, electron gun on north +, electron gun OP south side of detector).
140
impact
of
side of detector;
Volume
22. number
I
I
1
I
CHEXIICAL
I
I
I
I
I
PHYSICS
LETTERS
IS Srptentbzr
1973
I
7
J
20
40 ELECTKON
60. BEAM-ION
EC 100 120 :I;0 CETECTOR PNCLE (c?ql
(60
Fig. 5. Angular distribution of 0.8, 2.0, 3.0, and 5.0 cV O’ions from impact of 50 cV electrons on 02. Points sreaverugcs of data obtained \vith the cleFtron gun located on :hc t\vo symmetric opposite sides of the apparnruq with respect to the ion dctec:or.
of the B ‘2; state of 0:. Dunn’s selection rules for dipolar transitions between states of diatonic molecules [9] imply that near threshold * transitions from the ground 3Ti state of 0, to a ‘Xi state are sllowed for orientations of the internuclea;axis of the oxygen molecules both parallel to and perpendicular to the momentum of the input electrons. This would produce an isotropic angular distribution. Danby and Eland [lo] have recently made photoelectron-photoion coincidence measurements on 0,. They observed vibrational structure in the photoelectron spectrum corresponding to predissociation of the 3XS state and
socintion
0
,
2
3
4 FRAGMENT
5
6
ENERGY
7
e
9
10
(eV)
Fig. 4. Energy spcctr~~of osygpn fragments from dissociation of 02. A. This work. 50 cV electron impact. O+dctcctcd; B, Kicffcr ct al. [4]. 35 eV electron impact; C, Prcund [ 61, 39 cV clectrcn impact, high Rydbergs detected; D, Doolittlc ct al. [ 81, 34.6 CV photons, 0’ detected.
3.2. 0+/o,
were
Fig. 4 sl~ows an energy
spectrum
o f0 ’ ions obtained
at an electron ener_p of 50 eV. It is compared with an energy spectrum of Of ions obtained by Kieffer et al. [4] using the DKB apparatus, energy spectra of Of ions produced by photoionization [8] , and 0 atoms in high Rydberg states produced by electron impact on O2 [6]. All four sets of experiments show qualitatively similar general features and good agreement in the kinetic energies corresponding to the peak positions. Fig. 5 shows angular distributions of the four O+ peaks within the 30” to 115” region for E, = 50 eV. All four peaks show substantially isotropic distributions. Both Doolittle et al. [8] and Freund [6] have considered possible states OF the 0: core leading to the observed dissociation fragments-All the measurements, including the isotropic angular distribution, and our own appearance potential measurements appear consistent with the suggestion [8] that the 0.8 eV peak arises from predis-
able
to show
rhnt
the
photoions
in cokcidencc
possessed kinetic energies consistent with this prcdissociation. Assignments of 0; levels corresponding to tiic 7 and 3 eV peaks are more speculative. The states c4C; and 211,111 are the only likely candidates [5. 81. Transitions to the c4C; state would, near threshold, tend to produce an Of angular distritution peaked in the forward and backward directions with respect to the electron beam [9] if dissociation were immediate. Iiowcver. there is evidence from a number of sources (see ref. [6] ) for predissociation from this state above its first vibrational level. If the times involved in this were long enough (of the order of a rotation time), a more isotropic distribution would result. Excitation of the III,, state on with
these
electrons
* That is, for inpul electron energies such that the wavrfunctions of the two outgoing eleclrons from the process e + 0: O+ + 0 + c + e are nearly spherically symmetric
207
-
‘Volume 22. number
1
CHEhlICAL PHYSICS LETTERS
theother hand should lead to an 0’ angular distribution peaking near 90”, provided predissociation does not occur. If the isotropic distribution observed here persists to near threshold for excitation to these two states (approximately 25 eV). wc must invoke one of three possibilities: (i) involvement of both states in such a way that an isotropic O+ djstribution results *; (ii) predissociation involving iong cross-over times to repulsive states; or (iii) inclusion of non-dipolar terms in calculating the dependence of transition probability on tile orientation of the molecule with respect to the electron beam. Angular distribution measurements closer to threshold are being pursued. Freund [G] has suggested that the approximately 5 eV kinetic energy neutral oxygen peaks observed by him in time-of-flight spectra may arise from initial ization of 0, to the *Xi and/or 4Zi states formed
ionby
removal of the 0~23 electron. From Dunn’s arguments [9], we might expect an isotropic O+ angular distribution, such as that observed! From excitation to either of these states.
* Accurate measurements ~C;LTthe thresholds may permit a test of rhis possibility.
for these states
15 September
1973
Acknowledgement We are grateful to Dr. R.J. van Brunt for several very helpful discussions and to Drs. R.N. Compton and C.E. Klots of this laboratory for their constant support and encouragement.
References [l] N. Snsaki and T. Nakao, Proc. Imp. head. (Tokyo) 11 (1935) 138; 17 (1941) 75. [2] G.H. Dunn and L.J. Kicffer, Phys. Rev. 132 (1963) 2109; R.J. van Brunt and L.J. Kicffer, Phys. Rev. A 2 (1970) 1293. [3] L.J. Kicffcr nnd R.J. van Brunt. J. Chcm. Phyr. 4b (1967) 272B. r41 L..y. Kieffer, G.Af. La\~rcncc ancl J.hI. Slnrer, Proceedings VII ICPEAC. Amsterdam (1971) p. 574. (51 D. Rapp. P. Erqlandcr-Golden and D.D. Brigha, J. Chcm. Phys. 42 (1965)4081; R.J. van Brunt, R.C. Hirsch and W.D. Whitehead, RuII. _i,m. Phys. sot. 17 (1972) 1145. 161 R.S. Freund, J. Chem. Phys. 54 (1971) 3125. [71 J.T. Tate and W.15’.Lazier. Phys. Rev. 39 (1932) 254. 181 P.II. Doolittle, R.I. Schoen and K.E. Schubert. J. Chern. Phys. 19 (1968) 5108. G.11. Dunn. Phys. Rev. Lcttcrs 8 (1962) 62. Cl. Danby and J.H.D. Eland. Intern. J. Mass Spectrom. Ion Phys. 8 (1972) 153.